<!DOCTYPE html>
<html lang="zh_cn">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <h1>同源策略</h1>
    <button>发送请求</button>
    <script>
        // 同源：协议，域名和端口号必须完全相同
        // 违背同源策略就是跨域
        const btn = document.querySelector('button');
        btn.onclick = function(){
            const xhr = new XMLHttpRequest();
            xhr.open('GET', '/data');
            xhr.send();
            xhr.onreadystatechange = function(data){
                if(xhr.readyState === 4){
                    if(xhr.status >= 200 && xhr.status < 300){
                        console.log(data);
                    };
                };
            };
        };
    </script>
</body>
</html>